Apparatus for bleaching wood pulp



March 31, 1970 F. NARDI APPARATUS FOR BLEAOHING WOOD PULP 2 Sheets-Sheet 1 Filed July 25, 1966 INVENTOR.

March 31, 1970 F. NARDI 3,503,346

I APPARATUS FOR BLEACHING WOOD PULP Filed July 25, 1966 v 2 Sheets-Shet 2 INVENTOR.

I MM/ 2.446

fl /x466 United States Patent 3,503,846 APPARATUS FOR BLEACHING WOOD PULP Franco Nardi, Torviscosa, Italy, assignor to S.A.I.C.l. Societal Agricola Industriale per la Cellulosa Italiana S.p.A., Milan, Italy Filed July 25, 1966, Ser. No. 567,561 Claims priority, application Italy, Oct. 25, 1965, 23,815/ 65 Int. Cl. D21c 7/00 US. Cl. 162-236 6 Claims ABSTRACT OF THE DISCLOSURE An apparatus for bleaching a thick semi-chemical pulp having a concentration higher than 3549% of dry matter by the use of chemicals having an oxidizing action only having a stationary casing with one inlet for the thick pulp and at least one inlet for the chemicals. A chamber wherein the reaction is brought to completion has an outlet. A rotor is supported for rotation in the casing and has an impeller-like structure comprising two concentric sections, of which the inner one is fitted with centrifugating blades, while the outer section is fitted with a plurality of spaced pins of projections, designed to intercept the flow of air which is centrifugated by said blades. The maximum diameter of the outer section is at least one and a half times greater than the maximum diameter of the inner section. The chemical inlet opens into the chamber adjacent to the inner section of the rotor. The rotor is vertically journalled and the chamber is formed with an inlet flared axial port. A feeding unit is eoaxially secured to the rotor and extends partly into the axial port and is fitted with helical blades to obtain a first coarse fragmentation and a uniform feeding of thick pulp into the chamber.

This invention relates to an equipment for the processing of pulps obtained from woods, and possibly from other vegetals, as e.g. cane, bagasse and the like, in order to have same pulps efliciently bleached, by means of chemicals, by which an oxidizing action only is exerted, as e.g. the mixtures of chemicals, wherein hydrogen peroxide is present.

The equipment according to the invention is particularly designed for processing pulps of the so-called semichemical type, and is intended to have thick and very thick semi-chemical pulps thoroughly mixed with chemicals by which an oxidizing action only is exerted, in bleaching processes of the type as disclosed and claimed in the copending Patent Application Nardi et al. Process for Bleaching of Pulps Obtained From Vegetal Essences, Ser. No. 5 67,616 filed July 25, 1966. In said processes, the pulps obtained from vegetal sources, and in particular the semi-chemical pulps, are processed under conditions of high consistency, corresponding to concentrations of an order in the range of 40% and more of dry matter contents, thereby to obtain material advantages from both technical and economical viewpoints, as stated in more details in said co-pending patent application.

The main object of this invention consists in the provision of an equipment, that is designed to exert a mechanical action both on the pulp to be bleached, and which is fed thereto under said conditions of high consistency, and to the oxidizing chemicals, that are utilized in a liquid condition, thereby to obtain a thorough and intimate mixing of all particles and fibres of the pulp, with said chemicals, and thus to ensure a uniform and eflicient reaction in practically all points of raw pulp, in order to obtain a uniformly bleached pulp, suitable for making sufiiciently and uniformly white papers and pasteboards.

A further object of the invention is the provision of an equipment as stated above, that is designed to exert mechanical actions and effects as required for the intended purposes, without causing changes in the fluidity degree of pulp, i.e. without any modification in the Schopper-Riegler number.

The equipment according to the invention is substantially designed in such a manner as to have both the pulp and the chemicals simultaneously and uniformly acted upon, in order to obtain the splitting of pulp into elemental fibers and small fiber bundles, or at any rate into minute particles, while the chemicals are nebulized, whereby the thus dispersed pulp and the nebulized chemicals are brought and maintainedeven if for a very short timein suspension-in a violently agitated and vorticated atmosphere, whereby to ensure a substantially uniform distribution of said particles and of said chemicals throughout said atmosphere.

In fact, it has been ascertained that, as an effect of a transitory, and even very short dispersion and uniform mixing of pulp particles and of chemicals, all pulp fibers are brought into contact with same chemicals. Then, the pulp fibers are collected and left to rest in a substantially undisturbed room, as e.g. a chamber or tower wherein the reaction is completed. It has been observed that, at the end of said reaction, a throughout bleaching of pulp is obtained, even if the pulp has subsequentlyv turned into curds, and a subsequent opening of such curds and clots proves that the required degree of stable bleaching has been attained and maintained even in the inside thereof.

In more details, the main object of this invention consists in the provision of an equipment which is substantially characterized in that it comprises mechanical means, fitted whithin a chamber, that is formed with inlet ports for the thick pulp and for the liquid chemicals, as well as with outlet ports for the pulp fibres whereon and/or between which the chemicals are uniformly dispersed, being said means designed to cause a nebulization of said chemicals, a very fine dispersion of pulp, and a strong whirling motion of the air in the chamber, wherein the pulp fibres and the chemicals are thus suspended under conditions of substantially uniform distribution.

In a preferred embodiment form of the invention,such mechanical means consist of a rotary unit comprisive of centrifugating devices, splitting devices and whirling devices. Said centrifugating devices are on turn comprisive of a crown of centrifugating blades, located in the cen tral section of said rotary unit, and that is essentially realized under the form of a centrifugating rotor, to which the thick pulp is axially fed. The splitting and whirling devices consist on turn of projections, e.g. under the form of pins, arranged in rings in the outer section of same rotary unit, and which peripheral speed'is consequently higher than that of centrifugating blades. The air current, wherein the discrete pulp fibers and the nebulized chemicals are kept in a substantially uniform dispersion, is discharged through one, or possibly more .ports, which are tangentially directed in respect of said peripheral portion.

According to an important feature of the invention, the chemical inlet ports are located and directed in such a manner as to have same chemicals fed directly to the depression and action zone of the centrifugating blades, by which therefore, the task of nebulizing same chemicals is also performed.

It has been ascertained that the most favourable degree of thorough mixing of the thick pulp andthe chemicals, can be attained by having recourse to wide piping and ports for the feeding of chemicals, thereby preventing all drawbacks that are associated with the use of Spraying or nebulizing nozzles which, under the surrounding and operating conditions of the considered equipment, would undergo too heavy limitations and too frequent cloggings, owing in particular to the presence of greater amounts of silicates, and of further substances and compounds, leading 'to the quick formation of deposits and scales.

According to an advantageous feature of the invention, an auxiliary crown of blades is provided on the perimetraljportion of the rotary unit, in order to complete the centrifugating action exerted on the air current, wherein the pulp fibers and the nebulized reagents are uniformly suspended. The outer edges of such blades, or at least of one 'of them (that extend close to the inner wall of chamber or casing within which said rotary unit is fitted) are provided with bits of a very hard material, e.g. of high speed steel, carbides or the like, in order to have all scales which may form on said inner wall, continuously scraped off.

According to a further feature of the invention, said rotary unit is fitted in such a manner as to be turned round a vertical axis, and the thick pulp is axially fed from above thereto. The rotary unit is moreover comprisive of a feeder, revolvingly fitted in the axial bore through which the pulp is fed, and equipped with spiral blades, that are designed to perform a twofold task, i.e. to make the feeding of pulp into the mechanical processing chamber more regular and uniform, and to bring about a first, gross fragmentation of the large clots of pulp (which is previously expressed in order to bring it to the required concentration) thereby preventing possible cloggings, or at any rate possible troubles in the feeding.

The above and further features, effects and advantages of the invention will be better understood from a consideration of the following detailed description of a preferred embodiment form thereof, as shown in the accompanying drawings, wherein:

FIG. 1 is a side view of an equipment according to the invention, that forms a part of a plant of the type as disclosed in the previously stated co-pending patent application.

FIG. 2 is a view, partly sectioned on a diametral plane, of the main operating unit of said equipment.

FIG. 3 is an exploded view of same unit, which main components are shown in views part sectioned on the planes and lines IIIIII-III in the FIG. 4.

FIG. 4 is an axial view from above, partly'sectioned on the horizontal plane IVIVIV in the FIG. 2, of same unit. As shown in the FIG. 1, the equipment, indicated in its entirety by the letter D, is comprisive of a casing or shell, that is suitably supported, e.g. through a hollow column C, by a base B, wherein the prime mover is fitted. Such prime mover may be e.g. an electric motor, as required for driving the hereinafter described mechanical components of said equipment. The drive may be advantageously effected by a belt transmission, that can be properly adjusted by a belt stretcher, operated by a handwheel V. The different components, that are designed to perform merely structural or driving tasks, shall not be described in more detail, since they are already known, and are not covered by the invention.

The casing or stator of the equipment D is approximately similar to the casing of stator of already well known centrifugal pumps, and is formed with an axial inlet port E, and a tangential outlet port U, as well as with a flare-shaped section S, showing a number of inlet ports P, arranged on a circle, and designed for the introduction of required chemicals in the form of a liquid, and preferably also of steam.

Said equipment D -is located at the outlet end of a channel for feeding the thick pulp (that is obtained, 'g'

.4 by the action of a rotary multiplate press, as diagrammatically shown in the figure), while the tangential outlet port U is connected with the inlet port I, at the top of chamber or tower T, wherein the reaction is completed, and which dome is connected, by a recycling duct R, with said feeding channel A, whereby to have the air blast (wherein a part of pulp fibers and of nebulized chemicals is possibly yet suspended) properly recycled, said air blast being produced by the same equipment.

As shown in the FIGS. 2 to 4 inclusive, the considered equipment is comprisive of a rotary unit or rotor 10 (see FIG. 3), running inside said casing, that is made-up by a bottom section 11, having an inner, shallow circular chamber, and by a top section 12, having a cylindrical channel 13, that is connected with said inlet port E, a flare-shaped portion S with the pipe connections for the inlet ports through which the chemicals and steam are fed, and an outer flange 14 by which said top section is tightly secured to the outer edge 15 of bottom section 11. The mouth of the tangential outlet passage U (see FIG. 3) is formed as in 17 on the essentially cylindrical inside wall 16 of the circular chamber.

Said inlet passages P for the chemicals and the steam, have their outlets 18 (see FIGS. 2 and 3) in the stator inside, on the inner wall 19 of the flare-shaped section S, and thus in a zone wherein a depression is prevailing when the rotor 10 is turning.

Said rotor 10 essentially consists of a discoid body 20, having a central hub 21, by which it is fitted on the top end 22 of driving shaft 23, which is revolvingly supported within the hollow column C. Said body 20 is upwardly flared, and is connected with a tapered ogival component 24, which rotates along with the rotor, and that extends for a given length inside the cylindrical inlet channel 13, and the component 24 is fitted with a plurality of spiral blades 25. The unit 24, 25 is designed to ensure a uniform and regular feed of the thick pulp into the chamber wherein the rotor is operating, and to provide also for a first coarse fragmentation of larger crumbs, which may be formed as a consequence of the water squeezing action which is being exerted directly upstream of the considered equipment.

As clearly shown in FIGS. 3 and 4, a plurality of blades 26 are fitted in the central section of the rotor, and in particular near the flare-shaped portion 21 of disk 20. Thus, the structure and operation of said rotor central portion are essentially similar to those of an impeller of a centrifugal pump or fan. As shown in particular in FIG. 2, the outlets 18 of the chemicals and steam inlet passages P, are located and directed in such a manner as to bring the chemicals and steam directly under the action of the centrifugating blades 26. Fitted on the flat outer edge of disk 20 are a plurality of projections, preferably arranged in more concentric rows or'crown, and consisting of metal pins 27, having preferably but not necessarily a polygonal cross-section, e.g. a square cross-section, and with diagonals oriented so as to form tangents to their rotary motion. Finally, extending from the outer edge of the disk 20 are auxiliary blades 28, by which the centrifugating action of the air current, and of the particles suspended therein, is completed, and this air is also urged into the outlet passage U. Bits of a hard material are fitted on the outer edge of at least one of said blades 28, whereby to have the scales and deposits that may be progressively formed on the stator inner wall 16, continuously scraped-off.

It has been ascertained that an efiicient centrifugating action, associated with a whirling action in the centrifugated air current, can be obtained in a particularly advantageous manner, when a suitable ratio is maintained between the diameter of the rotor portion wherein the centrifugating blades 26 are fitted, and the diameter of rotor section wherein the breaking pins 27 are present, thereby to have the peripheral speeds of said portions conveniently differentiated. In fact, the air threads which are tangential centrifugated by the blades 26, are intercepted and broken by the pins 27, that are running at a higher linear speed, whereby a uniformly distributed whirling condition is brought-up and maintained in the circular flow that is being centrifugated by said blades. Thus, e.g., the diameter of the outer pin row may be close to the double of the outer diameter of the row of blades 26. Consequently, the air threads and the particles that are kept suspended in the air flow produced by said blades are intercepted and broken by projections that are running at a much higher linear speed.

Though an accurate definition of the generation and evolution of phenomena occurring in the inside of the operating equipment cannot obviously be given, it can be however assumed that same phenomena will occur approximately in the hereinafter stated sequence:

The thick pulp that uniformly flows downward through the cylindrical channel 13, is cut into small chips when contacted by the top edges of centrifugating blades 26, by which the chemicals, flowing from 18 near same edges, are simultaneously nebulized.

A centrifugal condition is produced in the central section of the mechanical treatment chamber, by said blades 26, in an essentially smooth flowing current of air, wherein the pulp chips and the nebulized chemicals are suspended, being thus by the air flow tangentially urged beyond the side edge of the blades 26.

As a consequence of the above action, the annular zone above the outer flat portion of disk 20 is flooded by the air current carrying the pulp chips in suspension, whereby same current is beaten, in a plurality of points, by the pins 27, that are running at higher linear speed, whereby a large number of uniformly distributed whirls are produced in said current and consequently the further broken, small pulp chips to be processed, are thoroughly mixed with the nebulized chemicals.

The materials suspended in the air current is thus urged at a high speed into the outlet channel U, with the centrifugation completed by the perimetral blades 28.

Obviously, the different components are dimensioned as a function of the throughput of the plant wherein the equipment is designed to operate. Similarly, the r.p.m. of the rotor shall be selected in a suitable reversed ratio to diameters of their sections, in order to attain the most convenient tangential speeds. Thus, e.g., in an equipment wherein amounts of the order of 4000 kg./h. of dry materials are to be processed, rotors having a max. diameter of about 800 mm. can be fitted, and in such a case the centrifugating blades 26 may have an operating diameter of about 35 -400 mm., while the pins are arranged in more concentric rows, having diameters ranging from about 450 up to 700 mm. A rotor dimensioned as above, allows attaining the most convenient processing conditions when driven at a speed in the range of 1500-2000 r.p.m., and preferably of 1700 r.p.m.

Consequently, the air threads that are centrifugated by the blades 26 will flow at tangential speeds of an order of 30 m./sec., and they will be intercepted and beaten by pins 27, that are running at linear speeds up to 60 m./ sec.

What I claim is:

1. An apparatus for bleaching a thick semi-chemical pulp having a concentration higher than 3540% of dry matter by the use of chemicals having an oxidizing action comprising a stationary casing having an inlet for said pulp, and an inlet for said chemicals, a chamber in said casing wherein the reaction is brought to completion having an outlet; means for feeding the material in said chamber and providing a first coarse fragmentation, a rotor supported for rotation in said casing having an impeller-like structure comprising an inner section, an outer section concentric to said inner section, said inner section having centrifugating blades to ensure centrifugation where said blades operate, said outer section having a plurality of spaced peripheral pins to provide minute fragmentation of the centrifugated material and a finely subdivided swirling motion of the gaseous atmosphere in which the particles and chemicals are suspended; to secure a homogeneous distribution of particles of said pulp and said pins catching the gases and particles and tangentially centrifugating them into and through said outlet in said chamber.

'2. Apparatus according to claim 1, wherein the maximum diameter of said outer section is at least one and a half times greater than the maximum diameter of said inner section.

3. Apparatus according to claim 1, wherein said inlet for said chemicals opens into said chamber adjacent to said inner section.

4. Apparatus according to claim 1 wherein said rotor is vertically journalled, said inlet for said pulp is a flared axial port and said rotor comprises a feeding unit coaxially secured to said rotor extending partly into said axial port, and said feeding means comprises helical blades to obtain said first coarse fragmentation and a uniform feeding of said pulp into said chamber.

5. Apparatus according to claim 1 wherein said centrifugating blades are fitted on the outer edge of said inner section to provide a centrifugated gaseous flow in which the pulp particles and the nebulized chemicals are uniformly suspended, accelerated and sent into and through said outlet in said chamber.

6. Apparatus according to claim 5, wherein bits of a hard material are fitted on the outer edge of said outer section in order to remove the scales and hard deposits that are being progressively formed on the inner wall of said casing.

References Cited UNITED STATES PATENTS 3,194,540 7/1965 Hager 259-7 3,284,055 11/1966 Johansen 2597 3,337,191 8/1967 Gall 2597 635,480 10/1899 Heinrich 259l07 2,859,110 11/1958 Sanford 162-4 HOWARD R. CAINE, Primary Examiner US. Cl. X.R. 

